To my mind, the change of ownership and control, plus the completely different software (actually the same as this one - currently), meant that the change in 2005 was a second version. People regarded it as the new version at the time. There was also a completely new set of moderators, which included Bazza and Ettie. Ettie went on to the be the top "Global Moderator". Most of these moderators went across to version 3 as well.
Whether the point is relevant depends on how accurate you want to be. I suppose a detail like this is not really terribly important.
But, I was one of the earliest members of the first forum and I've been a member of every version so I have a perspective which make me think that this new forum is, in fact , the fourth version.